The present invention relates to a stabilized laser device in which laser light from a semiconductor laser or the like is automatically controlled and stabilized at a desired level.
A laser device, such as a semiconductor laser or the like, is used in an optical recording/reproducing apparatus for recording/reproducing information at high density or in an optical communication system. Light output from the laser device varies widely in accordance with external factors such as ambient temperature. The laser output also changes with degradation of the light source. Therefore, a laser device must have a light output stabilized device for stabilizing the laser output and improving reliability.
FIG. 1 shows a conventional stabilized laser device. Output light (from the rear surface of the laser, in this case) from semiconductor laser 301 is photoelectrically converted by photodiode 302, which produces a voltage signal. The output signal from photodiode 302 is supplied to the inverting input terminal of differential amplifier 303. The non-inverting input terminal of differential amplifier 303 receives reference voltage Vr as a preset value for output light. The output terminal of differential amplifier 303 is connected to window comparator 304. Window comparator 304 comprises comparators 304a and 304b which compare the output signal from differential amplifier 303 with preset upper and lower limits th.sub.u and th.sub.l. Comparison results are then supplied to up and down terminals U and D of up/down counter 307 through gate circuit 305, consisting of AND gates 305a and 305b. Gate circuit 305 also receives clock pulses from clock pulse generator 306. The count of counter 307 is supplied to drive transistor 313 through D/A converter 308 and amplifier 310. The collector current of transistor 313 is then supplied to semiconductor laser 301.
More specifically, in such a stabilized laser device, reference voltage Vr is preset corresponding to the desired output of laser 301. Amplifier 303 supplies the difference voltage between the output light from laser 301 and reference voltage Vr to comparator 304. Comparator 304 generates an output which corresponds to the polarity of the difference voltage between the output light from laser 301 and reference voltage Vr.
When the output voltage from photodiode 302 is smaller than reference voltage Vr, a detection signal "1" is supplied from comparator 304a to AND gate 305a and a clock pulse is supplied to up terminal U of counter 307 through AND gate 305a. Consequently, the count of counter 307 increases, and the drive current supplied from the collector of drive transistor 313 to laser 301 also increases. The output light from laser 301 is thus increased so that the output of the photodiode 302 equals reference voltage Vr. On the other hand, if the output voltage from photodiode 302 is larger than reference voltage Vr, a detection signal "1" is supplied from comparator 304b to AND gate 305b, and a clock pulse is supplied to down terminal D of counter 307 through AND gate 305b. Consequently, the count of counter 307 decreases, and drive current supplied from the collector of transistor 303 to laser 301 decreases. Thus, the output light from laser 301 decreases so that the output of the photodiode 302 equals reference voltage Vr. In this manner, since the drive current of laser 301 is controlled in accordance with the difference between its output and reference voltage Vr, the output light from laser 301 can be controlled in accordance with reference voltage Vr.
However, in the above conventional device, if preset reference voltage Vr is changed during use, a period from the time voltage Vr is changed to the time the drive current of laser 301 is stabilized (at a value corresponding to reference voltage Vr), corresponds to the magnitude of change of reference voltage Vr and the clock pulse frequency. In other words, if the magnitude of change of reference voltage Vr is large, time for approximating the drive current to a value corresponding to reference voltage Vr is long. Because of this, in conventional stabilized laser devices, changes in the laser drive current in response to changes in the preset reference value are slow. Therefore, laser output is too large or too small for a period of time until the drive current controls it to equal the preset value. For example, then, recording state is unstable when information is written in a recording medium.